The heart is the organ responsible for keeping blood circulating through the body. This task would not be possible if it was not for the action of valves. Four heart valves are key components that facilitate blood circulation in a single direction, and that the contraction force exerted by the heart is effectively transformed into blood flow.
Each time the heart contracts or relaxes, two of the four valves close and the other two open. There are two states of the heart: relaxed or contracted. Depending on the state of the heart, a heart valve has two specific functions: either to open smoothly without interfering blood flow or to close sharply to impede the flow in the opposite direction.
The anatomy of the heart allows it to simultaneously maintain the flow of the two major blood circuits in the body: pulmonary circulation and systemic circulation, which also includes the coronary circulation. This simultaneous action of keeping blood flowing through both circuits requires that the heart valves work in pairs, namely, the tricuspid and the pulmonary valve work together to direct the flow toward the lungs, and the mitral and aortic valves direct the flow toward the rest of the body including the heart.
From the two circulations, the systemic circulation is the one that demands most of the energy of the heart because it operates under higher pressures and greater flow resistance. Consequently, the left heart is more susceptible to valve disorders. This condition makes the aortic and mitral valves primary subjects of research.
According to the American Heart Association it is estimated that around 19,700 people in the United States die every year from heart valve disease, and another 42,000 die from different causes aggravated by valvular problems. During 1996, 79,000 heart valve replacements were carried out in the United States, a quantity that was reported to increase by 5,000 more replacements by 1997. Although improvement has been evident in this area of medical treatments, still a mortality rate between 30% and 55% exists in patients with valvular prostheses during the first 10 years after surgery.
The aortic valve, representing almost 60% of the valve replacement cases, is located at the beginning of the systemic circulation and right next to the coronary ostia. Once the aortic valve closes the oxygenated blood flows into the heart through the right and left coronary arteries.
The mitral valve, located between the left atrium and the left ventricle offers a different set of conditions. Although the mitral valve is not surrounded by any arterial entrances, it is located in a zone with greater access difficulties, and its anatomical structure contains a set of “leaflet tensors” called chordae tendinae.
The human application of prosthetic heart valves goes back to 1960 when, for the first time, a human aortic valve was replaced. Since then, the use of valvular implants has been enhanced with new materials and new designs.
The first mechanical valves used a caged-ball mechanism to control blood flow. Pressure gradients across the occluder-ball produced its movement to close or open the flow area. Even though this design performed the function of a valve, there were several problems associated with it: The ball geometry and the closing impact of the ball against the cage ring were both causes of large downstream turbulence and hemolysis. In addition to blood damage, obstruction to myocardial contraction and thrombogenic materials were also problems.
Several designs having new materials including disks or leaflets instead of balls, improved the hemodynamic performance and durability of the implants, but two critical aspects remain pending for better solutions: 1) the highly invasive surgery required to implant the prosthesis, and 2) the thrombogenic effect of the implant's materials.
Typically, mechanical heart valve prostheses are made from pyrolytic carbon or other prosthetic materials that require rigorous anticoagulant therapy because the risk of coagulation is higher over the surface of the prosthesis. The thrombogenic aspect has drawn the attention of many biomedical institutions towards the creation and study of more biocompatible materials.
Currently, prosthetic heart valve technology includes several designs with disks or leaflets integrated into a rigid stent. This rigid stent is generally surrounded by a sewing cuff which allows the surgeon to suture the interface between the cuff and the tissue. This procedure, however, is highly invasive and its materials generally have a negative thrombogenic effect.
Prosthetic heart valves with rigid stents require open heart surgery for implantation. During the implantation procedure the patient is maintained alive by a heart-lung machine while the surgeon sutures the device into the heart. Due to the highly invasive nature of this procedure, not all individuals suffering from heart valve disease are considered proper candidates.
In those cases where a heart valve replacement has been performed, the risk of coagulation of blood becomes higher over the surface of the prosthesis. Mechanical heart valve prostheses made from pyrolytic carbon or other prosthetic metals require rigorous anticoagulant therapy. Other prosthetic valves use animal tissues, with which the thrombogenic effect is not as severe as for other materials, but durability is noticeably lower. Specifically, prosthetic valves constructed using animal tissue are prone to hardening as a result of being rejected by the body. Such hardening and rejection can ultimately lead to less than optimal performance.